Claude API Not Working: Common Fixes

"The Claude API is not working" almost always resolves to one of five specific failures. Identify the status code first, then apply the matching fix.

1. 401 unauthorized

The key is missing, revoked, or sent in the wrong header. Send it as the x-api-key header, not as a bearer token, and confirm the key belongs to the workspace you are billing.

2. 429 rate limited

You have exceeded requests or tokens per minute. Read the retry-after header, reduce concurrency, and batch small calls instead of firing them in parallel.

3. 529 overloaded

This is service capacity, not your code — see Anthropic API 529 overloaded and retry with backoff.

4. Timeouts and hanging requests

Long generations exceed default client timeouts. Raise the client timeout and use streaming for anything over a few hundred output tokens.

5. Malformed request errors

Check that messages alternate correctly, that max_tokens is set, and that the model identifier is current. Deprecated model names return a 404-style error rather than a clear message.

6. Everything fails at once

That is an incident, not an integration bug. Check the status page and Claude not responding before rolling back a deployment.